Patent Literature 1 discloses a lighting device for supplying a constant electric current to a solid-state light emitting device such as a light emitting diode (LED). Various types of lighting devices having a dimming function have been proposed. The lighting device disclosed in Patent Literature 1 is capable of performing the dimming function via pulse width modulation (PWM) control through use of a direct current/direct current (DC/DC) converter.
Unfortunately, the lighting device disclosed in Patent Literature 1 has posed a problem of the PWM control. The problem is such that the lowest gradation value is relatively large which is displayed in accordance with an on-duty capable of maintaining the constant current operation (the lowest gradation value is luminance displayed in accordance with the on-duty that is the minimum one larger than zero). Note that the on-duty is a ratio of the duration of an operating state to the duration of a non-operating state. For the lighting device the luminance of which is controlled via the PWM control, the luminance decreases with decreasing on-duty.
For example, with the lighting device according to Patent Literature 1, the lowest gradation value is about 1% in accordance with the on-duty that can maintain the constant current operation in the PWM control. Accordingly, a very low on-duty of e.g. 0.2% is difficult to achieve. For this reason, in cases where the lighting device is used as a backlight of a liquid crystal display device, for example, an expression at such very low luminance is difficult to display.